i40e: Register auxiliary devices to provide RDMA

Convert i40e to use the auxiliary bus infrastructure to export
the RDMA functionality of the device to the RDMA driver.
Register i40e client auxiliary RDMA device on the auxiliary bus per
PCIe device function for the new auxiliary rdma driver (irdma) to
attach to.

The global i40e_register_client and i40e_unregister_client symbols
will be obsoleted once irdma replaces i40iw in the kernel
for the X722 device.

Signed-off-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>